TACTILE PAPILLAE OP STAMENS 581 



The thin-walled character of the papillae is rendered more conspicuous, 

 by the fact that the outer epidermal wall is generally thickest around 

 the base of each papilla. The cavity of the papilla is occupied by a 

 small prolongation of the protoplast. Clearly these minute tactile 

 organs hardly differ from the sensory pits of the Cucurbitaceae, except 

 in the fact that the thin membrane enclosing the protoplasmic process 

 is developed as a papilla, an arrangement which must obviously facilitate 

 stimulation considerably. The insensitive basal region of the filament 

 is devoid of epidermal papillae. 



In Opuntia vulgaris the staminal filaments are sensitive on all sides 

 and throughout their length ; here, however, the filament always curves 

 inwards, i.e. towards the style. In this case, again, the outer wall of 

 each of the elongated epidermal sensory cells is furnished with a small 

 conical papilla, which is usually situated near its middle, and less 

 frequently towards one end (Fig. 234 b). The papillae of the outer or 

 longer stamens diminish in size towards the base of the filament, finally 

 scarcely projecting at all, and thus approaching the condition of tactile 

 pits. Around the bases of the papillae, the outer walls of the sensory 

 cells are more or less strongly thickened, just as in the case of Portidaca. 

 The walls of the papillae themselves are extremely delicate, never 

 exceeding "6 to *8 ijl in thickness ; the cuticle is also very thin at these 

 points. On coming into contact with a solid body, therefore, the 

 papillae are readily pushed in, with the result that the underlying 

 ectoplast suffers great deformation. Very similar tactile papillae occur 

 on the irritable stamens of Cereus spinosissimus. 



In that very remarkable genus of Orchids, Catasetum. the column 

 of the male flower ends in a slender process, from which the single 

 anther is suspended by means of a stout filament of considerable 

 length. From the sides of the column project a pair of long, straight 

 or slightly curved thick structures with tapering ends ; each of these 

 really consists of a ribbon-shaped prolongation of the margin of the 

 column, which has become rolled up around its longitudinal axis into a 

 tubular body. The name " antenna," applied to these appendages by 

 Ch. Darwin, aptly expresses their function. For if the tip of one 

 of the antennae is gently touched with a bristle or needle, the 

 adhesive disc common to the two pollen-masses instantly separates 

 from the rostellum, with which it was previously in organic con- 

 nection ; the curved stalk joining the disc to the pollen-masses 

 straightens itself with a jerk, and the whole pollen-apparatus shoots 

 forward with great violence. The actual propulsion of the'pollinia is 

 not a manifestation of irritability, but depends upon the instantaneous 

 equalisation of tissue-tensions within the stalk, which behaves like a 

 bent spring that is suddenly released. But this movement cannot 



